This invention relates generally to single conductor well logging systems, and more particularly, to simultaneous logging systems of the type which employ a single conductor cable to supply power to the subsurface unit for the radiation detectors located therein and to transmit the detector output signals to the surface as well as casing collar signals indicative of the depth of the subsurface unit.
Single conductor logging systems are well known. For example, in Scherbatskoy U.S. Pat. No. 2,740,053, there is disclosed a single conductor simultaneous well logging system in which pulses from the top detectors are transmitted up the cable as voltage pulses of opposite polarity. However, when such a system is used to log deep wells, the transmitted pulses are severely attentuated by the series resistance and shunt capacity of the cable so that it is hard to get meaningful information from the pulses when they arrive at the surface. This attenuation of the pulse type signals occurs in deep wells, which may be up to 30,000 feet in depth even when only a single detector is used in the logging system since the voltage pulse applied to the bottom end of the cable must charge the capacity of the entire length of the cable in order to appear as a recognizable voltage pulse at the surface.
Also, the logging unit must be protected from the intense heat (up to 400.degree.F.) that is often encountered in deep wells without making the subsurface unit large and cumbersome. While some arrangements heretofore proposed have enclosed the detector and associated circuitry in a Dewar container within the subsurface housing, such a Dewar container prevents the dissipation of the heat generated by the detector and electronic circuitry positioned within this container. Therefore, the power dissipated by the circuitry within the Dewar container must be held to an absolute minimum to prevent the excessive buildup of heat within the Dewar container. If this is not done, the length of time that continuous logging measurements can be made will be severely limited. In deep wells where heavy drilling mud is used, a long time is consumed for the subsurface unit to fall to the bottom of the well. Accordingly, the subsurface unit may be called upon to withstand high temperatures for as much as ten hours without malfunctioning.
Finally, the varying resistance of the cable as the subsurface unit is lowered into the well causes variations in the power actually supplied to the subsurface unit, thereby necessitating some sort of voltage stabilization circuitry in this unit. Prior art systems have utilized a constant current regulated supply at the surface and the operator adjusts this supply at the surface to get a fixed value of current down hole irrespective of line length. Since the current applied to the line remains relatively constant even though the resistance of the cable varies as it is paid out over the pulley, a relatively constant voltage can be provided for the subsurface unit by passing the constant current through a resistor within the subsurface unit and utilizing the voltage thus developed to power the probe. This system has several disadvantages. Usually, constant voltage supplies utilizing a constant current source and a resistor dissipate far too much power to be included within the Dewar container and yet such a supply cannot itself operate satisfactorily at 400.degree. F. Furthermore, the high output impedance of the constant current source permits large voltage fluctuations to exist on the line. As a result, the logging signals from the subsurface unit appear across the cable as fluctuating voltage signals which tend to charge and discharge the shunt capacity of the line. This charging and discharging of the line serves to attenuate the logging signals and limits the maximum length of the line that may be used before the logging signals are obscured. On the other hand, if a constant voltage regulated supply is used at the surface, the voltage actually applied to the subsurface unit will vary excessively. Furthermore, a low impedance is presented to the cable by this supply which will further tend to attenuate signals transmitted over the cable.
In single conductor logging systems it is also desirable to convert the detector signals to telemetry signals of the type described in copending McLaughlin et al application Ser. No. 400,174 filed Sept. 20, 1973 now U.S. 3,529,749 and transmit these telemetry type signals to the surface for processing and recording. Such telemetry signals extend the maximum pulse rates from the detector that can be transmitted without causing pulse overlap in the cable with voltage pulse type transmission over the cable, but do not solve the problems resulting from the cable attenuation itself.